Semiconductor device with thick bottom metal and preparation method thereof

ABSTRACT

A semiconductor device with thick bottom metal comprises a semiconductor chip covered with a top plastic package layer at its front surface and a back metal layer at its back surface, the top plastic package layer surrounds sidewalls of the metal bumps with a top surface of the metal bumps exposing from the top plastic package layer, a die paddle for the semiconductor chip to mount thereon and a plastic package body.

FIELD OF THE INVENTION

The invention relates to a preparation method of a power semiconductordevice, in particular, the invention relates to a semiconductor devicewith a thick bottom metal and a preparation method thereof.

DESCRIPTION OF THE RELATED ART

Generally, the power consumption of the power device is large; thereforepart of metal electrodes of the device is exposed from a plastic packagematerial encapsulating the semiconductor chip in the device to improveheat dissipation and the electrical performance of the device. Forexample, a US patent publication 2003/0132531A1 has disclosed asemiconductor package 24 with an exposed electrode at the bottom of thesemiconductor chip applied in the surface mount technology. As shown inFIG. 1A, a power chip MOSFET (Metal-Oxide-Semiconductor Field EffectTransistor) 10 is located inside of a metal can 12 with its draincontact connected to the bottom of the metal can 12 via a layer ofconductive silver epoxy 14. Can 12 also includes two rows of projections22 located on two of its opposing edges, which are provided to makeelectrical contact with respective lands on a circuit board. As such thedrain of MOSFET 10 is electrically connected to its place within acircuit through the projections 22 of the metal can 12. On the otherhand, a source contact 18 and a gate contact 20 of the MOSFET 10 and theprojections 22 are located on a surface of MOSFET 10 opposing its draincontact. A low stress and high adhesive epoxy 16 is deposited to fill ina gap around the MOSFET 10 and the can 12. Although the heat dissipationof the package 24 is improved, the metal can 12 has high cost ofpreparation in the actual production and it is difficult to accuratelyplace the MOSFET 10 in the can 22.

In another existing power devices as shown in FIG. 1B, Bonding pads 35 aand 35 b are formed at the front side of the MOSFET 30 and an electrode33 at the back side of the MOSFET 30 is connected to the bonding pad 35a via an through via 32 filled with a conductive material. The MOSFET 30is encapsulated by a plastic package layer 36 and a plastic packagehousing 34. The bonding pads 35 a and 35 b are electrically connected toan external circuit through metal bumps 37. However, in this powerdevice, the electrode 33, which is a metal layer, is very thin withrespect to the thickness of the housing 34, therefore the heatdissipation is poor.

It is within this context that embodiments of the present inventionarise.

BRIEF DESCRIPTION OF THE DRAWINGS

As shown in attached drawing, the embodiment of the invention is moresufficiently described. However, the attached drawing is only used forexplaining and illustrating rather than limiting the scope of theinvention.

FIG. 1A-FIG. 1B are cross-sectional schematic diagrams of the existingsemiconductor devices;

FIG. 2A-FIG. 2J are schematic diagrams illustrating a process of forminga thin semiconductor chip with a top plastic package layer at the frontsurface and a back metal layer at the back surface;

FIG. 3A is a top view of a first lead frame;

FIG. 3B-FIG. 3E are cross sectional views of the lead frame in FIG. 3A;

FIG. 4A-FIG. 4D are cross sectional schematic diagrams illustrating amethod of forming a semiconductor device with thick bottom metal usingthe first lead frame in FIG. 3A;

FIG. 5A-FIG. 5C are schematic diagrams illustrating a alternative leadframe and the method of forming a semiconductor device with thick bottommetal using the alternative lead frame;

FIG. 6A-FIG. 6B are cross sectional schematic diagrams of asemiconductor device formed by the first lead frame in FIG. 3A;

FIG. 7A is a top view of a second lead frame;

FIG. 7B-FIG. 7E are cross sectional views of the lead frame in FIG. 7A;

FIG. 8A-FIG. 8D are cross sectional schematic diagrams illustrating amethod of forming a semiconductor device with thick bottom metal usingthe second lead frame in FIG. 7A; and

FIG. 9A-FIG. 9B are cross sectional schematic diagrams of asemiconductor device formed by the second lead frame in FIG. 7A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to a cross-sectional view as shown in FIG. 2A, a semiconductorwafer 100 generally includes a plurality of semiconductor chips and aplurality of scribe lines at its front surface, where each scribe lineis located between two adjacent semiconductor chips; therefore, thescribe line can be used as a reference target for cutting to separateindividual semiconductor chips from the wafer. Furthermore, a pluralityof metal pads 110 are formed at the front surface of each semiconductorchip for electrical connection to a power supply and the ground or aterminal in signal transmission with an external circuit, etc.

Commonly, a under bump metal (UBM) layer is formed on the metal pads 110(not shown), such as Ni/Au layer. Metal balls or bumps 111 are depositedon the metal pads 110, as shown in FIG. 2B, for example, the metal bumpcan be a tin soldering ball, or copper block in spherical, cylindricalor wedge shape and the like. A first plastic package material isdeposited to form a plastic package layer 120 covering the front surfaceof the wafer 100. The first plastic package material can be an epoxyresin and the like. In a preferred embodiment, the first plastic packagelayer 120 only covers the center area of the front surface of the wafer100, not the whole front surface, as shown in FIG. 2C. The cross area ofthe first plastic package layer 120 is also a circle as the wafer 100with the radius of the first plastic package layer 120 being smallerthan that of the wafer, as such a first ring area 103 not covered by thefirst plastic package layer 120 is formed at the edge of the frontsurface of the wafer 100, and both ends of each scribe line are extendedand exposed in the first ring area 103. As shown in FIG. 2C, the firstplastic package layer 120 is completely encapsulated the metal bumps111.

As shown in FIG. 2D, the first plastic package layer 120 is ground untilthe metal bumps 111 are exposed from the first plastic package layer120. The first plastic package layer 120 is cut along a straight lineformed by extending two ends of each scribe line exposed in the firstring area 103, as shown in FIG. 2E, forming a shallow groove used as areference line 121. The mechanical strength of the wafer 100 isincreased due to the physical support of the first plastic package layer120, so that the wafer 100 can be thinned without breaking. In FIG. 2F,the wafer 100 is flipped and the center area of the back surface of thewafer 100 is ground by a grinding wheel (not shown) to form a recessedarea 150 while the outermost area of the wafer 100 is not ground, thus asupport ring 100 a is formed at the edge of the back surface of thewafer 100. The radius of the recessed area 150 is smaller than that ofthe first plastic package layer 120, therefore the support ring 100 a ispartially overlapped with the first plastic package layer 120, as suchthe mechanical strength of the wafer is improved.

As shown in FIG. 2G and FIG. 2H, a metal layer 130 is deposited at theback surface of the thinned wafer, then, the support ring 100 a is cutoff, for example by laser cutting. As shown in FIG. 2I, the wafer 100 isflipped back and mounted with the metal layer is directly attached on anadhesive film 140. The first plastic package layer 120, the wafer 100and the metal layer 130 are thus singulated along the reference line 121forming notches 115 by a cutter blade 240, thus forming a plurality ofindividual semiconductor devices 200A, each of which includes asemiconductor chip 101 formed by cutting the wafer 100, a top plasticpackage layer 120′ covering at the front surface of the chip 101 formedby cutting the first plastic package layer 120, and a bottom metal layer130′ located at the back surface of the chip 101 formed by cutting themetal layer 130. As a result, each device 200A comprises a chip 101, atop plastic package layer 120′, a bottom metal layer 130′ and metalbumps 111 formed on the pad 110 of the chip 101, where the top plasticpackage layer 120′ surrounds the sidewalls of metal bumps 111 with thetop of the metal bumps 111 are exposed from the top plastic packagelayer 120′.

In FIG. 3A, the lead frame 3000 comprises an array of a plurality of diepaddles 300, where the adjacent die paddles 300 are connected togetherby one or more connecting parts 301 with the top surfaces of theconnecting parts 301 and the die paddles 300 being coplanar, where eachconnecting part 301 comprises a support part 302 extending in theopposite direction from the front surface of the lead frame 3000 withthe bottom surfaces 302 a of all support parts 302 are coplanar. FIGS.3B, 3C, 3D and 3E are cross sectional views of the lead frame 3000 alonglines AA, BB, CC and DD. The thickness T1 between the front and backsurface of the die paddle 300 is less than the thickness T2 between thetop and bottom surfaces of the support part 302.

In FIG. 4A, each semiconductor device 200A is mounted on the frontsurface of each dies paddle 300 with the back metal layer 130′ directlymounted on the front surface of the die paddle 300 using an adhesivematerial 305. As shown in FIG. 4B, the lead frame 3000 with the devices200A mounted on its front surface is flipped and then a first adhesivefilm 311 is attached on the bottom surface 302 a of each support part302. The lead frame 3000 with the devices 200A and the adhesive film 311is flipped back, and a second adhesive film 312 is attached on the topsurfaces of the top plastic package layer 120′ and metal bumps 111. Infact, in the step of plastic molding, the flat adhesive films 311 and312 are placed inside a mold cavity of the molding equipment against thetop wall and bottom wall of the mold cavity respectively. Then the leadframe with the devices 200A is placed inside the mold cavity, thus theadhesive films 311 is pressed to attach on the bottom surface of thesupport parts 302 of the connecting parts 301 and the adhesive film 312is pressed to attach on the top surfaces of the first plastic packagelayer 120′ and the metal bumps 111. A plastic package material is theninjected in the empty space between the adhesive films 311 and 312 andthen is cured by heating to form a plastic package body 307 as shown inFIG. 4C. The plastic package body 307 covers the die paddles 300, theconnecting part 301 and the sidewalls of the devices 200A with the uppersurface of the top plastic package layer 120′ and the metal bumps 111and the bottom surface 302 a of the support part 302 exposing from theplastic package body 307. The plastic package material in a molten statebefore curing is very easily invaded into the space between the uppersurfaces of the top plastic package layer 120′ and the metal bumps 111and the adhesive film 312 thus covering the top surfaces of the topplastic package layer 120′ and the metal bumps 111; therefore, theplastic package body 307 is slightly ground from its top surface toexpose the top surfaces of the top plastic package layer 120′ and themetal bumps 111. After the adhesive films 311 and 312 are removed, asshown in FIG. 4D, the plastic package body 307 and the connecting part301 between two adjacent die paddles 300 is cut off to separate theplastic package body 307, die paddles 300 and the semiconductor devices200A, each of which includes a chips 101 with the top plastic packagelayers 120′ and the back metal layers 130′, forming a plurality ofindividual semiconductor devices 350.

The adhesive film 311 can be attached on the bottom surface 302 a of thelead frame 3000 before the semiconductor devices 200A are mounted on thedie paddle 300 of the lead frame 3000. Alternatively, the adhesive film311 can be attached on the bottom surfaces 302 a inside the mold cavityafter the semiconductor devices 200A are mounted on the die paddles 300of the lead frame 3000 as described above. However, the adhesive film312 can only be attached on the upper surface of the top plastic packagelayers 120′ and the metal bumps 111 inside the mold cavity after thesemiconductor devices 200A are mounted on the die paddles 300 of thelead frame 3000 as described above.

FIG. 4D is a cross sectional view of the semiconductor devices 350 alongthe dotted line AA in the FIG. 3A and FIGS. 6A-6B are the crosssectional views of the semiconductor devices 350 along the dotted lineDD in FIG. 3A. A first plastic package body portion 307 a surroundingthe sidewalls of the semiconductor device 200A, i.e., surrounding thesidewalls of the semiconductor chip 101, the top plastic package layer120′ and the back metal layer 130′. In general, the width of the notch355, which depends on that of the cutting blade, is designed whether thesidewall of the die paddle 300 is covered with the plastic package body307 after the cutting. For example, in FIG. 6A, the plastic package body307 covered the sidewalls of each die paddle 300 is not completely cutoff, thus forming a second plastic package body portion 307 csurrounding the sidewalls of the die paddle 300, where the first plasticpackage body portion 307 a is thicker than the second plastic packagebody portion 307 c. The back surface of each die paddle 300 is alsocovered with a bottom plastic package layer 307 b. As shown in FIG. 6A,the first plastic package body portion 307 a, the second plastic packagebody portion 307 c and the bottom plastic package layer 307 b form acontinuance structure. In another embodiment, as shown in FIG. 6B, theplastic package body 307 covering the sidewalls of each metal die paddle300 in the semiconductor device 350′ is cut completely, therefore thesidewalls of the die paddle 300 is not covered by the plastic packagebody 307, and the first plastic package body portion 307 a is separatedfrom the bottom plastic package layer 307 b by the die paddle 300. Inthe embodiment shown in FIGS. 6A-6B, the die paddle 300 cannot be usedas the contact end in electrical connection to the external circuit. Inthis case, the semiconductor chip 101 can be a common draindouble-MOSFET type vertical power device.

In the embodiment shown in FIGS. 4A-4D the lead frame 3000 can beprepared by etching a metal panel with an original thickness T2 formingthe die paddles 300 of a thickness T1, where T2 is thicker than T1, assuch, the connecting part 301 and the support part 302 are in a“T-shaped” structure. In another embodiment shown in FIGS. 5A-5B, thelead frame 3000′ can be prepared by printing or punching a metal panelwith an original thickness of T1 at the connecting part locations. InFIG. 5A, the support part 3020 of the connecting part has a trenchstructure and includes a bottom part 3021 parallel to the die paddle 300and two sidewalls 3022 connected to the both sides of the bottom part3021, where the bottom part 3021 includes a bottom surface 3021 a. Thethickness T1 of the lead frame 3000′ is less than the distance T2between the bottom surface 3021 a and the top surface of the die paddle300. FIG. 5C is a top view of the lead frame 3000′. The support part3020 is then cut in a subsequent step similar as described above in FIG.4D.

FIGS. 7A-7E are schematic diagrams of a lead frame 4000 according toanother embodiment of the present invention. The top view of lead frame4000 is similar as that of the lead frame 3000 in FIG. 3. However, inthis embodiment, the thickness of the die paddle 300 in the lead frame4000 is equal to the distance T2 between the bottom surface 302 a of thesupport part 302 and the front surface of the die paddle 300. In otherwords, the back surface of the die paddle 300 is coplanar with thebottom surface 302 a. FIGS. 7B-7E are cross sectional views of the leadframe 4000 along the dotted lines AA, BB, CC and DD shown in FIG. 7A.The lead frame 4000 can be prepared by etching a metal panel withoriginal thickness of T2.

In this embodiment, a groove 300 a is formed at the edge of the backsurface of each die paddle 300, as shown in FIG. 7D-FIG. 7E. FIG.8A-FIG. 8D are the schematic diagrams of a method of preparing thesemiconductor device using the lead frame 4000, which is similar as thatshown in FIGS. 4A-4D. However, in this embodiment, the plastic packagebody 307 is not covered the back surface of the die paddle 300.Furthermore, in the step of separating individual semiconductor devices,the plastic package body 307 filled in the groove 300 a is cut offforming a ring plastic package body 307 d surrounding the sidewalls ofthe bottom portion of the die paddle 300. FIGS. 9A-9B are crosssectional views of a semiconductor device 450 along the direction of thedotted line DD in FIG. 7A. As shown in FIG. 9A, the final semiconductordevice 450 includes a first plastic package body portion 307 a coveringthe sidewall of the device 200A, i.e., covering the sidewalls of thechip 101, the top plastic package layer 120′ and the back metal layer130′, a second plastic package body portion 307 c covering the sidewallof the top portion of the die paddle 300, where the first plasticpackage body portion 307 a and the second plastic package body portion307 c are continuance with the first plastic package body portion 307 abeing thicker than the second plastic package body portion 307 c, and aring plastic package body 307 d surrounding the sidewall of the bottomportion of the die paddle is continuance to the second plastic packagebody portion 307 c. In other words, the first plastic package bodyportion 307 a, the second plastic package body portion 307 c and thering plastic package body 307 d form a continuance structure.Alternatively, as shown in FIG. 9B, the plastic package body 307covering the sidewall of the die paddle 300 is cut off completely, thusthe sidewall of top portion of the die paddle 300 is not covered withthe plastic package body 307 c. In this case, the semiconductor device450′ only includes the first plastic package body portion 307 a and thering plastic package layer 307 d, where the first plastic package bodyportion 307 a is separated from the ring plastic package layer 307 d bythe top portion of the die paddle 300. In this embodiment, the backsurface of the die paddle 300 is exposed as a contact end for electricaland mechanical connection to the external circuit. In a preferredembodiment, the chip 101 is a vertical power MOSFET with the currentflowing from the front to the back of the device or vice versa, wherethe plurality of metal pads 110 includes the source and gate electrodesand the bottom metal layer 130′ includes the drain electrode.

The above detailed descriptions are provided to illustrate specificembodiments of the present invention and are not intended to belimiting. Numerous modifications and variations within the scope of thepresent invention are possible. The present invention is defined by theappended claims.

The invention claimed is:
 1. A preparation method of a semiconductordevice with a thick bottom metal comprises the following steps:providing a lead frame including a plurality of die paddles, wherein theadjacent die paddles are connected to each other by one or moreconnecting parts, each connecting part comprises a support part arrangedat a back surface of the connecting part and extending along a directionaway from the back surface, wherein bottom surfaces of all support partsare substantially coplanar; mounting a semiconductor chip covered with atop plastic package layer at a front chip surface and a back metal layerat a back chip surface opposite to the front chip surface on a frontpaddle surface of each die paddle, wherein the back metal layer isdirectly attached onto the front paddle surface of the die paddle;forming a plastic package body with a plastic package material coveringthe die paddles, the connecting part and the semiconductor chip with thetop plastic package layer and the back metal layer, wherein an uppersurface of the top plastic package layer and the bottom surfaces of thesupport part are exposed from the plastic package body; and cutting offa portion of the plastic package body and connecting parts between theadjacent die paddles to separate individual semiconductor devices eachincludes a semiconductor chip with the top plastic package layer and theback metal layer, wherein a thickness of the die paddle is less than adistance between the bottom surface of the support part and the frontsurface of the die paddle such that the back surface of the die paddleis covered with a bottom plastic package layer formed by cutting theplastic package body covering the back surface of the die paddle.
 2. Themethod of claim 1 further comprises attaching a first adhesive film atthe bottom surface of each support part and a second adhesive film onthe upper surface of the top plastic package layer before forming theplastic package body and removing the first and second adhesive filmsafter forming the plastic package body.
 3. The method of claim 1 furthercomprising grinding from a top surface of the plastic package body to asurface coplanar with the upper surface of the top plastic package layerto remove the plastic package material covering on the upper surface ofthe top plastic package layer after forming the plastic package body. 4.The method of claim 1, wherein cutting off a portion of the plasticpackage body and a connecting part between the adjacent die paddlesforms a first plastic package body portion covering the sidewalls of thesemiconductor chip, the top plastic package layer and the back metallayer.
 5. The method of claim 4, wherein cutting off a portion of theplastic package body and a connecting part between the adjacent diepaddles forms a second plastic package body portion covering thesidewalls of the die paddle.
 6. The method of claim 4, wherein portionsof the plastic package body covering sidewalls of the die paddle arecompletely cut off such that the sidewalls of the die paddle areexposed.
 7. A preparation method of a semiconductor device with a thickbottom metal comprises the following steps: providing a lead frameincluding a plurality of die paddles, wherein the adjacent die paddlesare connected to each other by one or more connecting parts, eachconnecting part comprises a support part arranged at a back surface ofthe connecting part and extending along a direction away from the backsurface, wherein bottom surfaces of all support parts are substantiallycoplanar; mounting a semiconductor chip covered with a top plasticpackage layer at a front chip surface and a back metal layer at a backchip surface opposite to the front chip surface on a front paddlesurface of each die paddle, wherein the back metal layer is directlyattached onto the front paddle surface of the die paddle; forming aplastic package body with a plastic package material covering the diepaddles, the connecting part and the semiconductor chip with the topplastic package layer and the back metal layer, wherein an upper surfaceof the top plastic package layer and the bottom surfaces of the supportpart are exposed from the plastic package body; and cutting off aportion of the plastic package body and connecting parts between theadjacent die paddles to separate individual semiconductor devices eachincludes a semiconductor chip with the to plastic package layer and theback metal layer, wherein a thickness of the die paddle is equal to adistance between the bottom surface of the support part and the frontsurface of the die paddle with the back surface of the die paddle beingcoplanar with the bottom surface of the support part, and wherein theback surface of the die paddle is exposed from the plastic package body.8. The method of claim 7, wherein the die paddle further comprises agroove formed at an edge adjacent the support part at the back surfaceof the die paddle, said groove forming a ring surrounding a centralportion of the die paddle and being filled with plastic package materialafter forming the plastic package body; a ring of plastic package bodyis formed at the edge at the back surface of the die paddle after thecutting process.
 9. The method of claim 7, wherein the connecting partand the support part form a “T-shaped” structure.
 10. The method ofclaim 7, wherein the support part has a trench structure including abottom portion substantially parallel to the die paddle and twosidewalls connected at each side of the bottom portion.